1. Field of the Invention
The present invention relates to an electric motor and an apparatus for manufacturing an electric motor, and particularly to a technique for reducing cogging torque due to magnetic anisotropy of rolled sheet steel. The present invention is applicable to rotary electrical machines such as motors and generators.
2. Description of Related Art
In a motor comprising a rotor with magnets embedded and a stator with a coil part formed by winding a wire in slots arranged at regular intervals in the circumferential direction, and driven to rotate by torque produced by the magnets and reluctance torque produced by the coil, cogging torque which makes the rotor pulsate is produced when the rotor rotates.
When the magnetic flux lines produced by the magnets in the rotor form closed magnetic paths passing through teeth of the stator, the sum of electromagnetic attractive forces between the stator and the rotor in the direction of rotation, at each rotation angle, varies depending on the relative position of the rotor to the stator. Since the stator has alternating slots and teeth which are different in magnetic resistance, while the rotor passes one slot pitch in its rotation, the direction of the electromagnetic attractive forces changes between the rotor rotation direction and the opposite direction. Thus, torque varies with a frequency proportional to the number of magnet poles or the number of slots, to produce cogging torque.
Conventionally, in order to reduce cogging torque, efforts have been made to improve the rotor structure, the stator structure, and the accuracy of die assemblies for forming the rotor and the stator. For example, there is known a skew arrangement in which layers forming a rotor are stacked in the axial direction, successively displaced in the circumferential direction, namely the rotor rotation direction, equiangularly. This structure is intended to displace the relative positions of the respective magnet poles in the rotor to the teeth of the stator which they are passing by, in the circumferential direction, to thereby displace the positions at which the cogging torques exerted on the respective layers take the maximum value, in the circumferential direction. Also a multi-stage skew arrangement in which the rotor is divided into two or more stages in the axial direction is known.
By adopting the rotor and stator structure like this, cogging torque is reduced but still remains. The inventor of this application found that one of the causes of cogging torque is magnetic anisotropy of electromagnetic sheet steel used for forming the rotor and stator. Rotor cores and stator cores for the motor are made from nondirectional electromagnetic sheet steel. However, even the nondirectional electromagnetic sheet steel has different magnetizing properties in the direction parallel to the direction in which it was rolled (flatten by means of rolls) and in the direction perpendicular to it. This magnetic anisotropy of the electromagnetic sheet steel causes cogging torque.
The applicant of this application made a proposal for removing variation in rotation caused by magnetic anisotropy in a synchronous motor, in JP 10-66283A. JP 10-66283A notes the magnetic anisotropy of a laminated stator, and discloses a method of manufacturing a laminated stator which can reduce torque ripple in the synchronous motor utilizing this magnetic anisotropy.
However, what JP 10-66283A intends to remove is variation in rotation caused by torque ripple, which is different from cogging torque. The applicant of this application also filed a Japanese patent application published as JP 2005-65479A for a motor designed to reduce cogging torque due to the magnetic anisotropy of electromagnetic sheet steel, prior to this application.
Rotor cores and stator cores for forming a motor are formed by punching them from rolled electromagnetic sheet steel (hoop material). FIGS. 1a and 1b are diagrams for explaining the direction in which electromagnetic sheet steel (hoop material) is rolled (referred to as “rolling direction”). In FIGS. 1a and 1b, the longitudinal direction of rolled electromagnetic sheet steel (hoop material) 4 (direction of feeding it) is parallel to the rolling direction, and the width direction of the electromagnetic sheet steel (hoop material) 4 is perpendicular to the rolling direction.
Even the nondirectional electromagnetic sheet steel has different magnetizing properties in different directions, depending on the rolling direction (magnetic anisotropy). Accordingly, the magnets in the rotor produce different magnetic flux densities in different directions, depending on the rolling direction, which causes cogging torque.
In JP 2005-65479A, rotor cores and stator cores for a motor are manufactured by press working such that cores to be stacked have different rolling directions. By arranging that the cores stacked do not have the same rolling direction, cogging torque due to magnetic anisotropy is reduced.
FIG. 2 shows an example of rotor structure. In this example, a rotor is formed by stacking a laminated core #1 and a laminated core #2, each consisting of 10 core plates 1a stacked, and hence, the rotor is formed as a laminated core structure consisting of 20 core plates in all.
The rolling direction of the laminated core #2 (indicated by an arrow in the drawing) is at a specified angle to the rolling direction of the laminated core #1 (indicated by an arrow in the drawing).
FIG. 3 is a diagram for explaining how cogging torque is reduced. In FIG. 3, a waveform of cogging torque of 8 cycles per rotation is shown in solid line, and a waveform of cogging torque of 8 cycles per rotation having a phase difference of 180° with respect to the cogging torque waveform in solid line is shown in broken line. Thus, by adding cogging torques having a phase difference of 180°, the total cogging torque can be made zero.
In JP 2005-65479A, by stacking cores having different rolling directions so that cogging torques having the opposite phases are added together, the total cogging torque is reduced.
The inventor of this application found that in addition to the rolling direction, the stator structure can cause cogging torque. For the stator, apart from a slotless motor, a structure having slots and teeth is known.
In the stator having teeth, the direction of magnetic flux lines is influenced by a tooth so that the magnetic flux lines extend along the tooth. Thus, when the rolling direction does not agree with the direction (orientation) of a tooth, the magnetic flux lines extend along the tooth.
FIG. 4 is a diagram for explaining relation between the rolling direction, the direction (orientation) of a tooth and the direction of magnetic flux lines. In FIG. 4, a rotor 1 and a stator 2 are shown only in part, and the stator 2 has teeth 2c and slots 2f. Suppose that the rolling direction 10 of the stator 2 (direction in which the electromagnetic sheet steel forming the stator 2 was rolled) is at an angle θ to the direction of a tooth. Here, since air and copper exist inside the slot 2f, the part inside the slot 2f has a relative magnetic permeability of about 1. Meanwhile, the tooth 2e, which is made from electromagnetic sheet steel, has a relative magnetic permeability of about 2000. Thus, even when the rolling direction agrees with the direction of the slot 2f, magnetic flux lines do not extend through the slot 2f but extend along the tooth 2e, and hence not parallel to the rolling direction. This means that when the rolling direction is rotated as mentioned above, the reference position for the phase of cogging torque does not rotate correspondingly. Thus, the inventor of this application found that apart from the rolling direction, the stator structure causes cogging torque, and that if the rolling direction is rotated without taking account of the stator structure, cogging torque cannot be reduced satisfactorily.
FIG. 5 is a cogging torque waveform diagram for explaining how the teeth influence the cogging torque. In FIG. 5, waveforms of cogging torques having 32 cycles per rotation are shown. Specifically, a waveform of cogging torque of 32 cycles per rotation is shown in thick solid line, and a waveform of cogging torque in the case where the rolling direction is angularly displaced by a specified angle (here, 5.625°) and there is no influence of restriction of the magnetic flux by teeth is shown in broken line.
Since the phase difference between the cogging torque shown in thick solid line and the cogging torque shown in broken line is 180°, the total cogging torque can be reduced to zero by adding both cogging torques.
A waveform of cogging torque in the case where the rolling direction is angularly displaced by a specified angle (here, 5.625°) and there is influence of restriction of magnetic flux by teeth is shown in thin solid line. Since the magnetic flux are influenced by the teeth, the phase difference between the two cogging torques is not 180°. Hence, the total cogging torque cannot be reduced to zero by adding both cogging torques. Thus, it is difficult to reduce the total cogging torque satisfactorily by the angular displacement of the rolling direction.
FIG. 6 is a diagram showing relation between the phase of cogging torque and the rolling direction. This relates to the case where cogging torque has 32 cycles per rotation. The change of the phase of cogging torque repeats when the rolling direction changes by 360°/32=11.25°.
The number of cycles per rotation of cogging torque due to the magnetic anisotropy of the electromagnetic sheet steel depends on the number of poles in the rotor and the number of slots in the stator. The table below shows an example.
TABLE 1Number ofNumber ofNumber ofNumber ofcycles on stator,cycles on rotor,polesslotsNatural number nNatural number n10121012n = 1n = 120n = 28242424n = 3n = 18363236n = 3n = 140n = 5
According to the example shown in table 1, when the number of poles is 10 and the number of slots is 12, cogging torque exerted on the stator has 10 or 20 cycles per rotation, and cogging torque exerted on the rotor has 12 cycles per rotation. When the number of poles is 8 and the number of slots is 24, cogging torque exerted on the stator has 24 cycles per rotation, and cogging torque exerted on the rotor has 24 cycles per rotation. When the number of poles is 8 and the number of slots is 36, cogging torque exerted on the stator has 32 or 40 cycles per rotation, and cogging torque exerted on the rotor has 36 cycles per rotation.
If the rolling direction of the electromagnetic sheet steel forming the core can be rotated by the angle corresponding to each tooth so that the rolling direction agrees with the direction (orientation) of each tooth, the positional relation between the rotor and the teeth is equivalent to that when the rotor is rotated by the same angle, due to the symmetry between the rolling direction and the shape. Thus, the phase of cogging torque is not influenced by the teeth, so that the conventional phase relation can be applied.
FIGS. 7 to 10 are diagrams for explaining the case where the rolling direction is made to agree with the direction of a tooth. FIG. 7 shows relation between the phase and the rolling direction at angles at which the rolling direction agrees with the directions (orientations) of teeth. The angle (machine angle) of the rolling direction is plotted on the horizontal axis and the phase of cogging torque (in degrees) is plotted on the vertical axis. The diagram relates to the case where cogging torque has 32 cycles per rotation, and shows the above relation in the machine-angle range of 0° to 90°.
The phase of cogging torque changes from 0° to 360° in the machine-angle range corresponding to one slot (here, the width of the range is 11.25°=360°/32). The points in the diagram show the phases at the angles at which the rolling direction agrees with the direction of a tooth. For example, when the number of slots is 36, when the rolling direction is made to agree with the direction of a tooth of the machine angle 10°, the phase is P1. When the rolling direction is made to agree with the direction of a slot of the machine angle 20°, the phase is P2. These correspond to the case where the rotor is rotated just mechanically.
Regarding each slot, it is possible to make the rolling direction agree with the direction of a tooth. FIG. 8 shows the case where the rolling direction is made to agree with the x-axis direction. FIG. 9 shows the case where the rolling direction is rotated by 20° to agree with the direction of one of the teeth of the stator. FIG. 10 shows the case where the rolling direction is made to agree with the x-axis direction and the rotor is rotated by 20°. In FIG. 10, as the rotor rotates, the phase of cogging torque advances.
It is conceivable to stack the same number of cores as slots, such that the cores are rotated by the angle corresponding to one slot, successively. Even in this case, in each core, only some of the teeth have a direction that agrees with the rolling direction. The other many teeth have a direction that does not agree with the rolling direction. Thus, it is difficult to expect reduction of influence of the arrangement of the teeth.